PHOENIX -- A federal judge pushed back Thursday against a contention by the Obama Justice Department that a tough new Arizona immigration law set to take effect next week would cause "irreparable harm" and intrude into federal immigration enforcement.

"Why can't Arizona be as inhospitable as they wish to people who have entered or remained in the United States?" U.S. District Judge Susan Bolton asked in a pointed exchange with Deputy Solicitor General Edwin S. Kneedler. Her comment came during a rare federal court hearing in the Justice Department's lawsuit against Arizona and Gov. Jan Brewer (R).

Bolton, a Democratic appointee, also questioned a core part of the Justice Department's argument that she should declare the law unconstitutional: that it is "preempted" by federal law because immigration enforcement is an exclusive federal prerogative.

"How is there a preemption issue?" the judge asked. "I understand there may be other issues, but you're arguing preemption. Where is the preemption if everybody who is arrested for some crime has their immigration status checked?"

At issue in Thursday's hearing, argued in a tan-colored "special proceedings" courtroom" inside the federal courthouse, was whether Bolton would grant a preliminary injunction to stop the law from taking effect while the federal lawsuit proceeds.

As dozens of protesters marched outside, the hearing marked the first round in the Obama administration's effort to stop the state's crackdown on illegal immigration. The tension in the courtroom reflected a broader national debate over what has become a political divisive issue: whether police should have the power to question people they suspect are in the United States illegally.

"The regulation of immigration is unquestionably, exclusively, a federal power," Kneedler told a rapt courtroom. Brewer, whose fierce criticism of the federal lawsuit has helped her popularity at home, watched silently from the front row, drawing a "Good afternoon, Governor" from the judge.

Lawyers for Brewer argued with equal force that the legislation, scheduled to take effect July 29, is a legal expression of a sovereign state's right to secure its borders against a tide of illegal immigration. The federal government, the lawyers said, has failed to act.

"We keep hearing that we can't really do anything about these illegal aliens -- Arizona should just deal with it," said John J. Bouma, Arizona's lead attorney. "Well, the status quo is simply unacceptable."

The law, which Brewer signed in April, empowers police to question people they have a "reasonable suspicion" are illegal immigrants and to send them to federal authorities for possible deportation. President Obama has strongly condemned the law, and the Justice Department filed suit July 6, setting up an unusual clash between the federal government and a state over who should enforce the nation's immigration laws.

Bolton did not indicate how she might rule, saying only that she will take the matter "under advisement." But she did subject Justice Department lawyers to some pointed questions.

Kneedler responded to her query about why Arizona authorities don't have the right to be inhospitable to illegal immigrants by saying the law has given the state the power to enforce immigration law "in, frankly, an unprecedented and dramatic way."